Viewing background through alpha channel of foreground object
If you want the planes to be in 3D space, then you could try using a material like this on the plane with the green screen:



This mixes between a transparent shader and a emission shader based on the difference between the the second color input of the mix node and the pixels in the image. Pixels which are within a .07 tolerance of the green color in the mix node will be transparent, other pixels will be opaque.